Their ability to understand is much further developed than their ability to communicate, so she may well understand more than you give her credit for. It is really important you stay with her to give her reassurance. If you are really consistent and don't feed, she will get the picture pretty quickly. Its awful at the time when you're there and she's screaming for milk, so being really sure you're ready to make the change before you start and sticking to your plan is essential.